Metal oxide semiconductor (MOS) devices typically comprise a pair of ion implanted source/drain regions in a semiconductor substrate, a channel region separating the source/drain regions, and a thin gate oxide and a conductive gate comprising polysilicon or other conductive material formed above the channel region. In a typical integrated circuit, a plurality of MOS devices of different conductivity types, such as n-type and p-type, are formed on a common substrate.
A traditional approach to forming MOS devices of different conductivity types on a single substrate is illustrated in FIGS. 1A-1G. As shown in FIG. 1A, field oxide areas 115 are formed, as by local oxidation of silicon (LOCOS) or shallow trench isolation (STI), in semiconductor substrate 100, then a thin gate oxide 105 is thermally grown, and conductive gates 110, such as polysilicon, are formed. A photoresist mask M1 is thereafter formed on the areas to be subsequently implanted with p-type impurities, and substrate 100 is implanted, as by ion implantation, with n-type impurities NLDD to form lightly or moderately doped regions 120, also called "shallow source/drain extensions" (see FIG. 1B). Adverting to FIG. 1C, mask M1 is then removed, and the areas previously implanted with impurities NLDD are masked with photoresist mask M2. Substrate 100 is thereafter implanted, as by ion implantation, with p-type impurities PLDD to form lightly or moderately doped regions 125.
Next, as shown in FIG. 1D, sidewall spacers 130 are formed on the side surfaces of the gates 110, as by depositing a blanket layer of a dielectric material, such as silicon nitride, and anisotropically etching. A photoresist mask M3 is thereafter formed on the regions implanted with p-type impurities (see FIG. 1E), and substrate 100 is implanted, as by ion implantation, with n-type impurities NS/D to form source/drain regions 135, which include lightly or moderately doped regions 120. Adverting to FIG. 1F, mask M3 is then removed, and the areas previously implanted with impurities NS/D are masked with photoresist mask M4. Substrate 100 is thereafter implanted, as by ion implantation, with p-type impurities PS/D to form source/drain regions 140. Mask M4 is then removed, leaving the structure shown in FIG. 1G.
Source/drain implants NS/D, PS/D are typically implanted at a higher energy and dosage than lightly or moderately doped implants NLDD, PLDD, so source/drain implants NS/D, PS/D penetrate deeper into substrate 100 than lightly or moderately doped implants NLDD, PLDD. Additionally, sidewall spacers 130 prevent heavy source/drain implants NS/D, PS/D from entering substrate 100 adjacent to or under gates 110 to obtain the desired device performance characteristics. Thus, source/drain regions 135, 140 have a step corresponding to spacer 130.
Disadvantageously, the above-described methodology employs four photoresist masks (M1-M4), each of which requires the steps of spinning on the photoresist, exposing it with a stepper, developing the photoresist, and stripping off the mask after ion implantation. Each of these steps adds to the cost of the semiconductor device and decreases manufacturing throughput, and also subjects the device to additional handling, thereby increasing the likelihood of defects.
Moreover, masks M1-M4 are all "critical masks"; i.e., extremely complex and difficult to design and use. The large number of fine features required to form the masks challenge the capabilities of the photolithographic process necessary to implement them, thereby increasing manufacturing costs and reducing production throughput. As design rules are reduced to 0.18 .mu.m and under, e.g., 0.15 .mu.m and under, to meet increasing demands for miniaturization and higher circuit density, shrinking feature sizes cause masks such as M1-M4 to become even more difficult and costly to design and use.
Accordingly, there exists a need for a method of manufacturing MOS semiconductor devices with a reduced number of critical masks, thereby reducing manufacturing costs and increasing production throughput.